Optical isolator with reduced insertion loss and minimized polarization mode dispersion

ABSTRACT

An isolator is disclosed that features a single birefringent correction element. The correction element is configured to eliminate differential group delay and walk-off simultaneously.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/240,441, filed Oct. 13, 2000 (Attorney Docket No,NFCS-014P).

BACKGROUND

[0002] 1. Field

[0003] The present disclosure relates generally to fiber optics, and inparticular, to optical isolators.

[0004] 2. The Prior Art

[0005] 3. Background

[0006] The optical isolator is an element of modern opticalcommunication networks. Optical isolators allow light to travel in onedirection, while blocking light traveling in an opposite direction. Theever-increasing speeds of today's optical networks have placed higherperformance demands on optical isolators. Today, network speeds of 40Gb/s and higher are required for many applications. Polarization ModeDispersion (PMD), Polarization Dependent Loss (PDL), and insertion lossare important characteristics which must be minimized in any high-speedoptical communication system.

[0007]FIG. 1 shows a prior art optical isolator as described in U.S.Pat. No. 4,548,478 and assigned to Fujitsu Limited of Kawasaki of Japan.The optical isolator 100 of FIG. 1 includes an optical fiber 102 fromwhich an incident light beam 104 is launched into a first lens 106. Twobirefringent plates 108 and 112 are placed on either side of a 45°Faraday rotator 110 within the path of light beam 104. When light passesthrough the birefringent plate 108 in a forward direction (left toright), the angle of refraction of an ordinary ray (o-ray) and anextraordinary ray (e-ray) are different, so that a polarizationseparation is realized. The o- and e-rays are then directed into theFaraday rotator 110, where their planes of polarization are rotated 45°.The o- and e-rays are then directed into birefringent plate 112, whichis configured to transmit the e- and o-rays in a parallel manner. Theseparallel beams are then focused into optical fiber 120 by second lens118. However, light traveling in a reverse direction, (from right toleft) will have its e- and o-rays refracted in a different manner by thebirefringent plates, causing the rays not to be focused into opticalfiber 102 by first lens 104.

[0008] While the optical isolator 100 of FIG. 1 performs its intendedfunction, certain disadvantages have become evident. For example, thedisplacement of the e- and o-rays in space (known as walk-off)introduces insertion loss and Polarization Dependent Loss (PDL) into theisolator in the forward path. Additionally, the fact that the two beamsare traveling different optical paths results in the two beams havingdifferent velocities when passing through the isolator. This results inthe device not being PMD-free that may not be acceptable for modernoptical communication systems.

BRIEF DESCRIPTION

[0009] A portion of an optical isolator herein referred to as an opticalisolator core is disclosed which may include: a first polarizerconfigured to receive incident light traveling along a path and refractsaid incident light into o-rays and e-rays. A rotator is disposed alongthe path and configured to rotate the polarization planes of the o-raysand e-rays. A second polarizer is disposed along the path and has anoptic axis 45° apart from, and a wedge cutting direction aligned as in,the first polarizer.

[0010] A correction element of birefringent material having a length andan optical plane within the optic axis of the second polarizer isprovided. The correction element has an optical axis angle and lengththat are chosen to compensate for PMD and walk-off introduced by thefirst and second polarizers.

[0011] An additional aspect of the disclosed optical isolator core isprovided which includes a first polarizer configured to separate lightincident in the forward direction into at least one o-ray and at leastone e-ray; a polarization rotator; a second polarizer; and a correctionelement having a crystal optic axis which lies in a plane defined by theat least one e-ray and said at least one o-ray.

[0012] A further aspect of the disclosed optical isolator core isprovided in which the at least one o-ray and at least one e-ray travelthrough the isolator separated by a predetermined walk-off distance. Thecorrection element is configured to substantially reduce the walk-offdistance between the at least one o-ray and said e-ray exiting thesecond polarizer. Additionally, the correction element is configured tosubstantially eliminate the first order polarization mode dispersion,namely DGD (Differential Group Delay).

[0013] Further aspects of the disclosed optical isolator core includethe o-ray and one e-ray intersecting at an angle β within the correctionelement. The correction element has a physical length of L. Thedisclosed optical isolator may be configured so the o-ray and e-ray exitthe second polarizer separated by a walk-off distance that isapproximately equal to the length L multiplied by the tangent of angleβ.

[0014] The correction element of the disclosed optical isolator may havea tangent of angle β defined as:${\tan (\beta)} = \frac{\left( {n_{e}^{2} - n_{o}^{2}} \right){\sin (\alpha)}{\cos (\alpha)}}{{n_{o}^{2}\sin^{2}\alpha} + {n_{e}^{2}\cos^{2}\alpha}}$

[0015] A method for receiving light passing through an optical isolatorin a forward direction through the disclosed isolator is disclosed. Themethod may comprise separating the light traveling in a forwarddirection into at least one o-ray and said at least one e-ray; rotatingthe polarization of the o-ray and one e-ray; refracting the o-ray andthe e-ray such that they are in substantially parallel paths; andpassing the o-ray and the e-ray through a correction element having anoptic axis in a plane defined by the substantially parallel o-ray ande-ray exiting the second polarizer.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0016]FIG. 1 is a diagram of a prior art optical isolator.

[0017]FIG. 2 is a functional diagram of the disclosed optical isolatorcore.

[0018]FIG. 3 is a detailed diagram of a correction element.

[0019]FIG. 4 is an isometric diagram of a correction element.

[0020]FIG. 5 is a functional diagram of the disclosed optical isolatorcore operating as an isolator showing light traveling in the reversedirection.

[0021]FIG. 6 is a diagram of a complete isolator unit.

DETAILED DESCRIPTION

[0022] Persons of ordinary skill in the art will realize that thefollowing description is illustrative only and not in any way limiting.Other modifications and improvements will readily suggest themselves tosuch skilled persons having the benefit of this disclosure.

[0023]FIG. 2 is a diagram of an improved isolator core 200 which showsincident light being applied to isolator core 200 along a path 204.Isolator core 200 includes a first polarizer 206 having a wedge angleθ₁. Isolator core 200 is preferably disposed within path 204. In oneaspect of the disclosed isolator core, the wedge angle ₁ of polarizer206 ranges from approximately 0° to approximately 20°. Typically, awedge angle of approximately 8° is used for high birefringence materialssuch as YVO₄, and TiO₂, and approximately 13° to 15° for lowbirefringence materials such as LiNbO₃. Polarizer 206 also has an opticaxis C₁ having an angle γ₁.

[0024] Polarizer 206 may be fabricated from birefringent materials knownin the art. Preferred materials include LiNbO₃, YVO4, and TiO₂ such thatthe polarizer 206 will separate the incident light into o-rays (shown asa solid line for the condition where n_(e)>n_(o)) and e-rays (shown as adashed line) as is known in the art.

[0025] Isolator core 200 also includes a rotator 208 disposed withinpath 204 and configured to receive the o- and e-rays from polarizer 206.Rotator 208 may comprise any non-reciprocal optical element known in theart such as a garnet Faraday rotator for rotating the planes ofpolarization of the incident o- and e-rays at a predetermined angle,such as approximately 45°.

[0026] Isolator core 200 further includes a second polarizer 210disposed within path 204. Polarizer 210 also has an optic axis C₂ havingan angle γ₂. In one aspect of a disclosed isolator core, the angle γ₂ ofpolarizer 210 is approximately 45° apart from the angle γ₁ of thepolarizer 206.

[0027] Polarizer 210 may be fabricated from any birefringent materialknown in the art, such as LiNbO₃, YVO₄, and TiO₂. The polarizers 206 and210 are preferably formed from the same material.

[0028] Polarizer 210 is disposed in path 204 to receive the o- ande-rays from rotator 208, and is optically configured using methods knownin the art such that when the o- and e-rays exit, they are refracted andaligned in a substantially parallel manner.

[0029] The two polarizers may have optic axes C1 and C2 that are 45°apart. Additionally, the difference between the two optic axes may equalapproximately 45°. In another aspect of the disclosed isolator core, theangles θ of both polarizers 206 and 210 are substantially equal.

[0030] Isolator core 200 further includes a correction element 212,shown in more detail in FIG. 3, having a length of L, and an optic axisC having an angle α. Correction element 212 is disposed in path 204 toreceive the o- and e-rays from the polarizer 210 of FIG. 2. Correctionelement 212 may be fabricated from birefringent materials known in theart, such as LiNbO₃, YVO₄, and TiO₂.

[0031] Correction element 212 may be optically configured according tothe diagram of FIG. 3 and the equations below. The incident o- ande-rays are separated by a walk-off distance d when they are received bycorrection element 212, and are refracted such that a predeterminedangle β is formed.

[0032] Additionally, referring to FIG. 3, by optimizing the optic axisangle α and the length L, both the PMD and the walk-off can be correctedat the same time while the o- and e-rays may be recombined at a distanceL. As can be seen by inspection of FIG. 3, the e-ray and o-ray may berefracted by correction element 212 such that the rays intersect at apoint proximate to the distal face 215 of correction element 212. It isto be understood that the correction element 212 may comprise a widevariety of shapes other than that shown in FIG. 3. In one aspect of thedisclosed isolator core, the faces 213 and 215 are substantiallyparallel.

[0033]FIG. 4 is a three dimensional diagram of a correction element 212.FIG. 4 is provided to show how the optical plane P and the angle α ofthe optic axis C of correction element 212 are configured in one aspectof the disclosed isolator core. As can be seen by inspection of FIG. 4,the optical plane P of correction element 212 is preferably chosen suchthat it lies in a plane formed by the incident o- and e-rays. The angleα of optic axis C preferably lies within the optical plane P. Theoptical plane P may be aligned with or perpendicular to the optic axisof the second polarizer 210.

[0034] Correction element 212 may be configured by utilizing thefollowing equations:

[0035] The tangent of angle β as shown in FIG. 2 may be found from:${\tan (\beta)} = \frac{\left( {n_{e}^{2} - n_{o}^{2}} \right){\sin (\alpha)}{\cos (\alpha)}}{{n_{o}^{2}\sin^{2}\alpha} + {n_{e}^{2}{\cos^{2}(\alpha)}}}$

[0036] The relationship between the walk-off distance d, length L, andthe angle β can be found from:

d=L·tan (β)

[0037] The PMD may be found from:${PMD} = \frac{{{n_{e}^{\prime}(\alpha)}L} - {n_{o}L}}{c}$

[0038] where c is the speed of light in vacuum, and where:${n_{e}^{\prime}(\alpha)} = \frac{n_{o}n_{e}}{\sqrt{{n_{o}^{2}\sin^{2}\alpha} + {n_{e}^{2}\cos^{2}\alpha}}}$

[0039] We can solve the above equations to solve for the desiredvariable.

[0040] For example, L≈0.2 to 0.5 mm, and α=10° to 15°.

[0041]FIG. 5 is a diagram of an isolator core 500 showing the isolatorcore functioning as an isolator when light is incident in the reversedirection, traveling from right to left. As can be seen by inspection ofFIG. 5, when light is incident from the right, the o- and e-rays willnot be recombined when they exit polarizer 206.

[0042]FIG. 6 is a diagram of a isolator 600 including first polarizer206, rotator 208, second polarizer 210, and correction element 212forming an isolator core as shown and described above.

[0043] Isolator 600 may further include a first collimator 604 having afiber pigtail 606 and a coupling lens 608, and a second collimator 605having a fiber pigtail 612 and a coupling lens 610, all of which may beformed from materials known in the art. It is contemplated that anyoptical fibers known in the art may be utilized with the disclosedoptical isolator.

[0044] It is further contemplated that the disclosed isolator may befabricated in a wide variety of advantageous manners. For example, theisolator 600 may also include a magnetic ring enveloping the firstpolarizer 206, rotator 208, second polarizer 210, and correction element212, further defining an isolator core. The magnetic ring may be formedfrom materials known in the art. Finally, the isolator 600 may beencapsulated in an outer housing and sealed as is known in the art.

[0045] The above equations and disclosed aspects result in an opticalelement in which the walk-off distance may be kept to a minimum, therebyminimizing polarization dependent loss. Furthermore, the correctionelement of the present disclosure allows for the e- and o-rays to traveloptical paths that are substantially equal in length, further reducingthe effects of PMD and DGD as well as reducing insertion loss.

[0046] It is contemplated that the disclosed optical isolator andisolator core may be advantageously deployed in a variety ofapplications where low-loss elements are needed. For example, thedisclosed isolator may be used in critical long-haul applications suchas optical amplifiers, where low PMD and DGD are critical. Thecorrection element of the present disclosure may also be advantageouslyused in other passive optical components such as circulators andintegrated polarization beam splitters and combiners.

[0047] While embodiments and applications of this disclosure have beenshown and described, it would be apparent to those skilled in the artthat many more modifications and improvements than mentioned above arepossible without departing from the inventive concepts herein. Thedisclosure, therefore, is not to be restricted except in the spirit ofthe appended claims.

What is claimed is:
 1. An optical isolator core comprising: a firstpolarizer configured to receive incident light traveling along a pathand refract said incident light into o-rays and e-rays; a rotatordisposed along said path and configured to rotate the polarizationplanes of said o-rays and e-rays; a second polarizer disposed along saidpath and having an optic axis approximately 45° apart from said firstpolarizer and having a wedge cutting angle substantially the same assaid first polarizer; and a correction element of birefringent materialhaving a length and an optic axis having a cutting angle, wherein saidlength and said optic axis angle are chosen to compensate fordifferential group delay and walk-off introduced by said first andsecond polarizers.
 2. The isolator core of claim 1, wherein said firstand said second polarizers each have approximately the same wedge angle.3. The isolator core of claim 2, wherein said first polarizer has anoptic axis angle of approximately +/−45°.
 4. The isolator core of claim3, wherein said second polarizer has an optic angle of approximately 0°or 90°.
 5. The optical isolator of claim 1, wherein a distance traveledby said o-rays and said e-rays through said correction element is equalto said length of the correction element multiplied by the tangent ofsaid predetermined angle.
 6. The optical isolator of claim 1, whereinsaid correction element further includes an optical plane in which saido-rays and said e-rays travel, wherein said optical plane is alignedwith or perpendicular to said optic axis of said second polarizer. 7.The optical isolator of claim 1, wherein said correction elementcomprises a single piece of material.
 8. The optical isolator of claim1, wherein said correction element is configured such that said e- ando-rays are refracted such that said e- and o-rays intersect at a pointproximate to a distal face of said correction element.
 9. An opticalisolator adapted for receiving light transmitted through the isolator ina forward direction comprising: a first polarizer configured to separatelight incident in the forward direction into at least one o-ray and atleast one e-ray; a polarization rotator; a second polarizer; and acorrection element having a crystal optic axis which lies in a planedefined by said at least one e-ray and said at least one o-ray.
 10. Theoptical isolator of claim 9 wherein said at least one o-ray and said atleast one e-ray travel through said isolator separated by a walk-offdistance and said correction element is configured to substantiallyeliminate said walk-off distance between said at least one o-ray andsaid e-ray exiting said second polarizer.
 11. The optical isolator ofclaim 9 wherein said correction element is configured to substantiallyeliminate differential group delay.
 12. The optical isolator of claim 9wherein said first polarizer has a crystal optic axis angle ofapproximately +/−45°.
 13. The optical isolator of claim 9 wherein saidsecond polarizer has a crystal optic axis angle of approximately 0° or90°.
 14. The optical isolator of claim 13 wherein said correctionelement has a crystal optic axis α which lies with the plane defined bysaid at least one o-ray and said at least one e-ray.
 15. The opticalisolator of claim 9 wherein said correction element has a length L and acrystal optic axis angle α which are selected such that said at leastone e-ray is refracted by said correction element such that therespective light paths of said e- and o-rays intersect at a locationproximate to a face of said correction element.
 16. The optical isolatorof claim 15 wherein said o-rays and said e-rays are refracted by saidcorrection element.
 17. The optical isolator of claim 15 wherein said atleast one o-ray and said at least one e-ray intersect at an angle β. 18.The optical isolator of claim 15 wherein said at least one o-ray andsaid at least one e-ray exit said second polarizer separated by awalk-off distance which is approximately equal to said length L of thecorrection element multiplied by the tangent of angle β.
 19. The opticalisolator of claim 18 wherein said tangent of angle β is defined as${\tan (\beta)} = \frac{\left( {n_{e}^{2} - n_{o}^{2}} \right){\sin (\alpha)}{\cos (\alpha)}}{{n_{o}^{2}\sin^{2}\alpha} + {n_{e}^{2}\cos^{2}\alpha}}$


20. The optical isolator of claim 9, wherein said first and secondpolarizers comprise birefringent material.
 21. The optical isolator ofclaim 9, wherein said first polarizer, said polarization rotator, saidsecond polarizer, and said correction element are arranged in a sequencealong an axis of said isolator.
 22. An optical isolator adapted forreceiving light transmitted through the isolator on a forward directioncomprising: a first polarizer configured to separate light incident inthe forward direction into at least one o-ray and at least one e-ray; apolarization rotator; a second polarizer configured to refract at saidat least one o-ray and at least one e-ray exit such that they exit saidsecond polarizer in substantially parallel light paths separated by awalk-off distance; and a correction element having a length and acrystal optic axis which lies in a plane defined by said at least oneo-ray and at least one e-ray, and wherein at least one of said at leastone o-ray and at least one e-ray exiting said second polarizer arerefracted by said correction element such that their respective lightpaths intersect at an angle β.
 23. The optical isolator of claim 22wherein said correction element is configured to substantially eliminatesaid walk-off distance between said at least one o-ray and at least onee-ray exiting said second polarizer.
 24. The optical isolator of claim22 wherein said correction element is configured to substantiallyeliminate differential group delay.
 25. The optical isolator of claim 22wherein said first polarizer has a crystal optic axis angle ofapproximately +/−45° relative to a beveled of said first polarizer. 26.The optical isolator of claim 22 wherein said second polarizer has acrystal optic axis angle of approximately 0° or 90° relative to abeveled of said second polarizer.
 27. The optical isolator of claim 22wherein said polarization rotator comprises a 45° Faraday rotator. 28.The optical isolator of claim 22 wherein said correction element has alength L and a crystal optic axis cutting angle (x which are selectedsuch that said at least one o-ray or said at least one e-ray arerefracted by said correction element such that their respective lightpaths intersect at a location proximate to a face of said correctionelement.
 29. The optical isolator of claim 22 wherein both of said atleast one o-ray or said at least one e-ray are refracted by saidcorrection element.
 30. The optical isolator of claim 22 wherein said atleast one o-ray and said at least one e-ray intersect at an angle β. 31.The optical isolator of claim 30 wherein said at least one o-ray andsaid at least one e-ray exit said second polarizer separated by awalk-off distance which is approximately equal to said length Lmultiplied by the tangent of angle β.
 32. The optical isolator of claim31 wherein said tangent of angle β is defined as${\tan (\beta)} = \frac{\left( {n_{e}^{2} - n_{o}^{2}} \right){\sin (\alpha)}{\cos (\alpha)}}{{n_{o}^{2}\sin^{2}\alpha} + {n_{e}^{2}\cos^{2}\alpha}}$


33. The optical isolator of claim 22, wherein said first and secondpolarizers comprise birefringent material.
 34. The optical isolator ofclaim 22, wherein said first polarizer, said polarization rotator, saidsecond polarizer, and said correction element are arranged in a sequencealong an axis of said isolator.
 35. A method for receiving light passingthrough an optical isolator in a forward direction through the isolatorcomprising: separating the light traveling in a forward direction intoat least one o-ray and said at least one e-ray; rotating thepolarization of said at least one o-ray and said at least one e-ray;refracting said at least one o-ray and said at least one e-ray such thatthey are in substantially parallel paths; and passing said at least oneo-ray and said at least one e-ray through a correction element having anoptic axis in a plane defined by said substantially parallel at leastone o-ray and said at least one e-ray exiting said second polarizer. 36.The method of claim 35 wherein said correction element is configured tosubstantially eliminate said walk-off distance between said at least oneo-ray and at least one e-ray exiting said second polarizer.
 37. Themethod of claim 35 wherein said correction element is configured tosubstantially eliminate the first order polarization mode dispersion,namely DGD.
 38. The method of claim 35 wherein said correction elementhas a length L and a crystal optic axis cutting angle α which areselected such that said at least one o-ray and said at least one e-rayare refracted by said correction element such that their respectivelight paths intersect at a location proximate to a face of said ocorrection element.
 39. The method of claim 38 wherein said at least oneo-ray and said at least one e-ray exit separated by a walk-off distancewhich is approximately equal to said length L multiplied by the tangentof angle β.
 40. The method of claim 39 wherein said tangent of angle βis defined as${\tan (\beta)} = \frac{\left( {n_{e}^{2} - n_{o}^{2}} \right){\sin (\alpha)}{\cos (\alpha)}}{{n_{o}^{2}\sin^{2}\alpha} + {n_{e}^{2}\cos^{2}\alpha}}$


41. An optical isolator comprising: means for separating light travelingin a forward direction into at least one o-ray and said at least onee-ray; means for rotating the polarization of said at least one o-rayand said at least one e-ray; means for refracting said at least oneo-ray and said at least one e-ray such that they are in substantiallyparallel paths; and means for passing said at least one o-ray and saidat least one e-ray through a correction element having an optic axis ina plane defined by said substantially parallel at least one o-ray andsaid at least one e-ray exiting said second polarizer.
 42. The opticalisolator of claim 41 wherein said correction element is configured tosubstantially eliminate said walk-off distance between said at least oneo-ray and at least one e-ray exiting said second polarizer.
 43. Theoptical isolator of claim 41 wherein said correction element isconfigured to substantially eliminate the first order polarization modedispersion, namely DGD.
 44. The optical isolator of claim 41 whereinsaid correction element has a length L and a crystal optic axis cuttingangle α which are selected such that said at least one o-ray and said atleast one e-ray are refracted by said correction element such that theirrespective light paths intersect at a location proximate to a face ofsaid correction element.
 45. The optical isolator of claim 44 whereinsaid at least one o-ray and said at least one e-ray exit separated by awalk-off distance which is approximately equal to said length Lmultiplied by the tangent of angle β.
 46. The optical isolator of claim45 wherein said tangent of angle β is defined as:${\tan (\beta)} = \frac{\left( {n_{e}^{2} - n_{o}^{2}} \right){\sin (\alpha)}{\cos (\alpha)}}{{n_{o}^{2}\sin^{2}\alpha} + {n_{e}^{2}\cos^{2}\alpha}}$